A gas turbine engine generally includes one or more compressors followed in turn by a combustor and high and low pressure turbines. These engine components are arranged in serial flow communication and disposed about a longitudinal axis centerline of the engine within an annular outer casing. The compressors are driven by the respective turbines and compressor air during operation. The compressor air is mixed with fuel and ignited in the combustor for generating hot combustion gases. The combustion gases flow through the high and low pressure turbines, which extract the energy generated by the hot combustion gases for driving the compressors, and for producing auxiliary output power.
The engine power is transferred either as shaft power or thrust for powering an aircraft in flight. For example, in other rotatable loads, such as a fan rotor in a by-pass turbofan engine, or propellers in a gas turbine propeller engine, power is extracted from the high and low pressure turbines for driving the respective fan rotor and the propellers.
It is well understood that individual components of turbofan engines, in operation, require different power parameters. For example, the fan rotational speed is limited to a degree by the tip velocity and, since the fan diameter is very large, rotational speed must be very low. The core compressor, on the other hand, because of its much smaller tip diameter, can be driven at a higher rotational speed. Therefore, separate high and low turbines with independent power transmitting devices are necessary to drive the fan and core compressor in aircraft gas turbine engines. Furthermore since a turbine is most efficient at higher rotational speeds, the lower speed turbine driving the fan requires additional stages to extract the necessary power.
Many new aircraft systems are designed to accommodate electrical loads that are greater than those on current aircraft systems. The electrical system specifications of commercial airliner designs currently being developed may demand up to twice the electrical power of current commercial airliners. This increased electrical power demand must be derived from mechanical power extracted from the engines that power the aircraft. When operating an aircraft engine at relatively low power levels, e.g., while idly descending from altitude, extracting this additional electrical power from the engine mechanical power may reduce the ability to operate the engine properly.
Traditionally, electrical power is extracted from the high-pressure (HP) engine spool in a gas turbine engine. The relatively high operating speed of the HP engine spool makes it an ideal source of mechanical power to drive the electrical generators connected to the engine. However, it is desirable to draw power from additional sources within the engine, rather than to rely solely on the HP engine spool to drive the electrical generators. The LP engine spool provides an alternate source of power transfer, however, the relatively lower speed of the LP engine spool typically requires the use of a gearbox, as slow-speed electrical generators are often larger than similarly rated electrical generators operating at higher speeds.
However, extracting this additional mechanical power from an engine when it is operating at relatively low power levels (e.g., at or near idle descending from altitude, low power for taxi, etc.) may lead to reduced engine operability. Traditionally, this power is extracted from the high-pressure (HP) engine spool. Its relatively high operating speed makes it an ideal source for mechanical power to drive electrical generators that are attached to the engine. However, it is desirable at times to increase the amount of power that is available on this spool, by transferring torque and power to it via some other means.
Many solutions to this transformation are possible, including various types of conventional transmissions, mechanical gearing, and electromechanical configurations. One such solution is a turbine engine that utilizes a third, intermediate-pressure (IP) spool to drive a generator independently. However, this third spool is also required at times to couple to the HP spool. The means used to couple the IP and HP spools are mechanical clutch or viscous-type coupling mechanisms.
U.S. Pat. No. 6,895,741, issued May 24, 2005, and entitled “Differential Geared Turbine Engine with Torque Modulation Capacity”, discloses a mechanically geared engine having three shafts. The fan, compressor, and turbine shafts are mechanically coupled by applying additional epicyclic gear arrangements. The effective gear ratio is variable through the use of electromagnetic machines and power conversion equipment.
High-speed electric machines are almost always manufactured with low pole counts, lest the magnetic materials experience excessive core losses at higher frequencies that results in an inefficient motor design. This is primarily related to the fact that the soft material used in the vast majority of present motors is a silicon-iron alloy. It is well known that losses resulting from changing a magnetic field at frequencies greater than about 400 Hz in conventional silicon-iron based materials causes the material to heat, frequently to a point where the device cannot be cooled by any suitable means.